In Wampler U.S. Pat. No. 5,695,471, a continuous flow pump of rotary design is disclosed, suitable for permanent implantation in humans, for use as a chronic ventricular assist device. The disclosed device uses passive, magnetic radial bearings to maintain an impeller and its support shaft for rotation about an axis, thus eliminating the necessity for a drive shaft seal.
Typically prior art blood pumps use mechanical bearings to support the rotor with respect to the stator. The use of hydrodynamic thrust bearings has been disclosed for aiding in preventing thrombosis. However, blood cell damage and various other problems may be created by the use of hydrodynamic thrust bearings and mechanical bearings and it is desirable for a blood pump to use a magnetically suspended rotor if possible. However, a problem associated with magnetically suspending the rotor is that movement of the pump may result in movement of the rotor in the axial direction, causing the rotor or the impeller to contact a part of the pump casing. It is highly undesirable in a blood pump construction to have any continuing contact between the impeller and a part of the blood pump casing.
In Wampler U.S. Pat. No. 5,695,471, there is disclosed the shuttling back and forth of the rotor assembly with each cardiac cycle of the user, in order to prevent thrombosis. If this axial shuttling would not happen naturally, it would be desirable to have a system in which it could be induced electromagnetically.
In Wampler U.S. application Ser. No. 08/910,375, filed Aug. 13, 1997, now U.S. Pat. No. 5,840,070, the use of hydrodynamic bearings in an implantable blood pump is disclosed. With the hydrodynamic bearings, there is a frictional engagement which has to be overcome before the hydrodynamic bearings can be fully effective. The frictional engagement is a result of a preload that is applied by the front loading of the magnetic bearing. Thus an initial starting force is required that will enable the initial frictional engagement to be overcome. This initial starting force can create difficulties, and it would be advantageous to avoid its necessity.
Due to the preload of the negative axial spring rate of the passive radial magnetic bearings of the rotor, there is an axial magnetic force urging the impeller rearward against the casing. In view of the foregoing, it would be advantageous to be able to manipulate the axial force on the impeller during the starting of the pump.
There have been attempts to alleviate the starting axial force problem by having suspended systems using electromagnets. Typically in such systems, a dedicated electromagnetic control system is used to control the position of the impeller. There would be separate dedicated electronic controls and electromagnets for controlling the position of the rotor and the impeller. However, these separate dedicated electronic controls and electromagnets add to the weight, volume and complexity of the system.
It is, therefore, an object of the present invention to provide a rotary blood pump in which the rotor is magnetically suspended.
Another object of the present invention is to provide a rotary blood pump in which contact between the impeller and the blood pump housing is avoided.
A still further object of the present invention is to provide a novel rotary blood pump in which the axial forces on the impeller may be controlled.
An additional object of the present invention is to provide a novel rotary blood pump in which the axial position of the impeller may be controlled in an efficient manner.
Another object of the present invention is to provide a novel rotary blood pump that is small, light, simple in construction, and relatively easy to manufacture.
Other objects and advantages of the present invention will become apparent as the description proceeds.